1. Field of the Invention
The present invention relates to techniques and methods for detecting and quantifying surface characteristics and material conditions using light scattering.
2. Description of the Related Art
Detecting and monitoring fatigue damage is needed for preemptive failure repair or replacement of critical components in engineering systems. A macroscopic crack does not generally manifest until very late in the life of the component high cycle fatigue (HCF) conditions. However, surface structural changes and micro-cracks can occur much earlier on the surface and could possibly be used as a precursor for fatigue crack formation that leads to catastrophic failure. Therefore, it is of great importance to develop methods to quickly detect localized deformation and micro-cracking early in the fatigue life, well in advance of micro-crack coalescence and macroscopic crack growth.
Early in the fatigue process, dislocations in crystalline solids migrate to give rise to localized deformation, which in turn leads to micro-crack initiation and eventual failure. Crack formation typically occurs very late in fatigue life (within the last 20%) under high cycle fatigue conditions and, therefore, early determination of fatigue and estimation of remaining life requires the detection of dislocation structures as they impinge on the surface of the specimen. Optical visualization of precursor dislocation structures prior to crack formation is generally not possible for engineering components. However, in materials that have moderate to high surface residual stresses, the initial formation and movement of dislocations can reduce surface residual stress. The relationship between cyclic loading and a decrease in residual stress has been observed and reported by many researchers. For example, substantial relaxation of residual stresses during fatigue loading has been observed using x-ray diffraction techniques. The relaxation of residual stresses by 30% in mild steel under fatigue loading conditions was also measured using high sensitivity Moiré interferometry. Similarly, a laser light scattering technique was shown to efficiently detect not only micro-cracks, but also a reduction in surface roughness on wire specimens that were subjected to high cycle fatigue conditions. This change in roughness was attributed to cyclic relaxation of large residual stresses at the surface that resulted from the wire drawing process.
Microcrack formation occurs relatively early in the life of samples under low cycle fatigue (LCF) conditions. The number of microcracks that develop prior to the formation of the primary crack also increases with the greater imposed strain amplitude under LCF conditions. Monitoring the gradual increase in the microcrack density early in the life of an LCF specimen can provide an accurate prediction of fatigue life. Although this monitoring can be performed using surface replication techniques, a less time intensive method is needed to make microcrack monitoring practical for life assessment of components in service.
Various techniques have been proposed for the optical measurement of surface roughness and defect detection including laser ultrasonic, laser Doppler vibrometry, interferometry, and scattered light scanning.
In view of the foregoing, there is a need in the art for methodology and system that utilize scattered light detection so as to be more reliable, simpler in operation, and have the lowest cost to implement when compared to conventional system.